Determining power over ethernet impairment

ABSTRACT

A system and method of determining an unbalanced current condition in Power over Ethernet applications are disclosed. In some implementations, a user or network administrator may be notified of potential impairments due to unbalanced current.

RELATED APPLICATION

This application claims is a continuation of and claims priority to U.S.Utility patent application Ser. No. 12/323,292 filed Nov. 25, 2008 whichclaims priority to U.S. Provisional Patent Application Ser. No.61/007,240 filed Dec. 11, 2007, the disclosure of which are incorporatedby reference herein in their entirety.

BACKGROUND

Aspects of the present invention relate generally to Power over Ethernettechnologies, and more particularly to a system and method ofdetermining an unbalanced current condition in Power over Ethernetapplications.

Recent technological and market developments have resulted in a growinginterest in Power over Ethernet (PoE) applications such that manynetwork equipment manufacturers and systems integrators are endeavoringto design and implement enhancements to PoE. PoE has been standardizedin a specification promulgated by the Institute of Electrical andElectronics Engineers (IEEE), specifically, the IEEE 802.3af standardfor providing power to data terminal equipment (DTE) via a mediumdependent interface (MDI).

In operation, PoE is similar to that of a traditional telephone networkin which operating power necessary for the electrical components in thetelephone is delivered from the central office through the telephonecable, i.e., it is not necessary to couple the telephone to anindependent external power source. In PoE implementations, power istypically delivered to DTE devices from Ethernet switches or powersourcing equipment (PSE) via the local area network (LAN) cablingitself. Operating power provided through the LAN cables is then employedto power Internet Protocol (IP) telephones, wireless access points,security or web cameras, and the like. This technology does not requirealteration of the Ethernet infrastructure, and eliminates therequirement that networked DTE devices be supplied with operatingcurrent from an independent external power source.

It is expected that the existing IEEE 802.3af standard will soon beaugmented by another specification, IEEE 802.3at (or PoE+), which isunder development. As currently contemplated, PoE+ will supportincreased current requirements, and accordingly, some of the challengesassociated with supplying direct current (DC) power over category 5(Cat5) or category 3 (Cat3) network cables will be exacerbated by thehigher current levels prescribed by IEEE 802.3at. One potentialimpediment is a current mismatch between the positive and negative (+/−)wires of a given twisted pair. In some instances where the current isnot equal, a net induced magnetic field can saturate transformers anddecrease effective open circuit inductance (OCL), thus causing droop andother signaling degradation. Various factors may influence such acurrent mismatch including, but not limited to, different respectiveresistances in the +/− wires, and different contact qualities or contactresistances at the connections. Regardless of the source of themismatch, however, the end result is the same; attendant signaldegradation can cause packet errors or even link instability or failure.

Hence, it may be desirable in some circumstances to provide a method andsystem that effectively identify an unbalanced current condition in PoEapplications.

SUMMARY

Embodiments of the present invention overcome the above-mentioned andvarious other shortcomings of conventional technology, providing asystem and method of determining an unbalanced current condition inPower over Ethernet applications. In some implementations, a user ornetwork administrator may be notified of potential impairments due tounbalanced current.

In accordance with one embodiment, a method of determining an impairmentin a Power over Ethernet application may generally comprise monitoringoperation of an echo canceller associated with a PHY device, determiningwhen an echo energy reflected back to the device is above a threshold,and triggering an alert responsive to the determination. Either a powersourcing equipment device or a data terminal equipment device may beconfigured to perform the forgoing method.

In accordance with another embodiment, a device for use in a Power overEthernet application may generally comprise: a transmitter; a receiver;an echo canceller to remove echo energy from a signal received at thereceiver; and a tap monitor to monitor operation of the echo canceller;wherein output from the tap monitor may be employed to trigger an alertresponsive to a determination that the echo energy is above a threshold.

The foregoing and other aspects of various embodiments of the presentinvention will be apparent through examination of the following detaileddescription thereof in conjunction with the accompanying drawingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram illustrating components of oneembodiment of a Power over Ethernet architecture.

FIGS. 2A and 2B are simplified graphs plotting tap adaptations in anecho canceller.

FIG. 3 is a simplified flow diagram illustrating general operation ofone embodiment of a method of determining signal impairment due to anunbalanced current condition.

DETAILED DESCRIPTION

While the possibility of unbalanced current resulting in transformersaturation in PoE implementations is generally known, no single solutionhas been adopted by the industry. Many solutions have been proposed toaddress this issue, and several are under active consideration at theIEEE 802.3at development meetings. The proposed solutions generallyinvolve additions or alterations of the magnetics or of the physical(PHY) layers of the power sourcing equipment (PSE) or the powereddevices (or data terminal equipment (DTE)). Additionally, most proposedsolutions waste power by implementing resistors added to the magneticspath. While potentially feasible, these strategies to eliminate or tominimize the effects of unbalanced current in PoE have associated costand size penalties that cannot be avoided. In contrast, embodiments ofthe invention set forth herein do not increase the size or the cost ofthe transformers and the PHY layer components, and will not waste anypower.

Turning now to the drawing figures, FIG. 1 is a simplified block diagramillustrating components of one embodiment of a Power over Ethernetarchitecture. In the FIG. 1 embodiment, PoE architecture 100 generallycomprises a PSE device 110 coupled to a DTE device 120 via a networkcable 190.

In accordance with one embodiment, the PHY layer connection between PSEdevice 110 or DTE device 120 and the line-side (i.e., the LAN cable,such as a Cat5 or Cat3 cable), as well as digital signal processing(DSP) information obtained during the link-up process, may be employedto determine if a transformer at the transmitting or the receivingdevice is saturated. In some implementations, hardware register settingsmay be accessed, for example, to determine that the transformer issaturated; additionally, certain hardware registers may be set orsoftware interrupts may be generated to indicate this condition. Basedon this information (i.e., hardware register settings or softwareinterrupts) or in accordance with another trigger mechanism, higherlevel software may alert a system user or network administrator thatcorrective action may be appropriate or required. Such corrective actionmay include ensuring that contacts are clean, changing the LAN cable, orreducing current levels.

PSE device 110 generally comprises PHY devices (i.e., PHY transmitters111A and 111B and PHY receivers 112A and 112B), an echo canceller 113,and a power source 119. PHY transmitters 111A and 111B and PHY receivers112A and 112B may be any PoE compliant PHY layer capable of full-duplexoperation and suitable for use in connection with relevant standardsincluding, but not limited to, IEEE 802.3ab, 802.3af, and 802.3at, aswell as other standards developed and operative in accordance with knownprinciples. The present disclosure is not intended to be limited to anyparticular PHY layer structure or architectural implementation.

PHY transmitters 111A and 111B may be generally operative to transmitdata signals to DTE device 120 via cable 190. In that regard, PHYtransmitter 111A may be coupled to a twisted pair of wires 191 and 192associated with cable 190 via a transformer 114A; similarly, PHYtransmitter 111B may be coupled to a twisted pair of wires 193 and 194associated with cable 190 via a transformer 114B. As illustrated in FIG.1, PHY receiver 112A may be operative to receive data signals from DTEdevice 120 over twisted pair of wires 191 and 192 associated with cable190 via transformer 114A, and PHY receiver 112B may be operative toreceive data signals from DTE device 120 over twisted pair of wires 193and 194 associated with cable 190 via a transformer 114B.

Output of power source 119 may be electrically coupled to the line-sideof transformers 114A and 114B as illustrated. In PoE applications, powersource 119 may be operative to supply 48 volts of electric potential inaccordance with the IEEE 802.3af standard, but other voltages may bedesirable in some circumstances. Accordingly, it is contemplated thatpower source 119 may be implemented to produce electric potentials lessthan or greater than a nominal 48 volts, depending upon the overalloperational characteristics or requirements of the system orcommunications protocol in connection with which PSE device 110 isintended to be used.

DTE device 120 generally comprises PHY transmitters 121A and 121B, PHYreceivers 122A and 122B, and a load 128. As described above withreference to PSE device 110, PHY transmitters 121A and 121B and PHYreceivers 122A and 122B associated with DTE device 120 may beimplemented as or generally comprise any PHY layer compatible with (orotherwise suitable for use in connection with) a desired communicationsstandard. As indicated in FIG. 1, PHY receiver 122A and PHY transmitter121A may be coupled to twisted pair 191, 192 via a transformer 124A, andPHY transmitter 121B and PHY receiver 122B may be coupled to twistedpair 193, 194 via a transformer 124B.

Load 128 may be electrically coupled to the line-side of transformers124A and 124B as illustrated. The depiction of load 128 in FIG. 1 isintended to represent any generic electrical load requiring power tooperate DTE device 120 for its intended purpose. Load 128 may generallyrepresent any electrical or electronic component such as an applicationspecific integrated circuit (ASIC) or a microprocessor, for example. Inthat regard, in the FIG. 1 embodiment, DTE device 120 is illustrated ascomprising a central processing unit 129, but the present disclosure isnot intended to be limited to any particular electrical component orstructural arrangement at DTE device 120.

For example, in some implementations, DTE device 120 may comprise aVoice over Internet Protocol (VoIP) telephone, a wireless (e.g.,wireless LAN or Bluetooth) router or access point, a security camera orbuilding access control system, a web camera, or some other electronicdevice requiring operating power. These various embodiments of DTEdevice 120 may have different components (such as microprocessors,memories, displays, or a combination of these and other components)requiring power, and these components and component combinations aregenerically illustrated in FIG. 1 as load 128. It will be appreciatedthat DTE device 120 may comprise additional components, such as DC/DCconverters, transformers, or voltage regulators, for instance, asnecessary or desired to control or otherwise to regulate the operatingvoltage supplied to load 128. These additional components may beselected, for example, depending upon the nature and operationalcharacteristics of load 128 and the overall design parameters of DTEdevice 120.

As noted briefly above, network cable 190 may generally comprise twistedpair 191, 192 and twisted pair 193, 194 that are operative to carry datasignals and operating power from PHY transmitter 111A associated withPSE device 110 to PHY receiver 122A associated with DTE device 120;similarly, additional twisted pair 193, 194 may be coupled between PHYtransmitter 111B associated with DTE device 120 and PHY receiver 122Bassociated with PSE device 110.

It is noted that the FIG. 1 embodiment represents only one example of atypical PoE architecture. A conventional Cat5 network cable generallycomprises four twisted pairs, only two of which are employed for10BASE-T and 100BASE-TX Ethernet communications. Thus, in an alternativeembodiment, two spare pairs of wires may be used to supply power in PoEapplications; this is in contrast to the FIG. 1 embodiment in which thesignal pairs (191,192 and 193,194, respectively) are used to deliverpower. While the general operation set forth below is applicable to PoEarchitectures using the signal wires for power transmission, those ofskill in the art will readily appreciate that the present disclosure isnot intended to be limited to the architecture illustrated in FIG. 1.

In operation, voltage supplied from power source 119 to transformer 114Ainduces currents i₁ and i₂ in wires 191 and 192, respectively. Ideally,currents i₁ and i₂ are matched or balanced, i.e., of equal magnitude,however, this condition is not always satisfied. For example, respectiveresistances R₁ and R₂ in wires 191 and 192, respectively, may differ forvarious reasons, causing a mismatch or unbalanced current condition inwhich i₁ is not equal to i₂. Similarly, the various PHY layers at PSEdevice 110 and DTE device 120 are coupled to cable 190 via an RJ-45connector, for example, in Ethernet implementations; an imperfectconnection caused by pin misalignment or soiled contacts may create asmall resistance, resulting in mismatched currents.

In an unbalanced current condition, the line-side coil on transformer114A carries a residual current equal to i₁−i₂ (as illustrated in FIG.1), which tends to induce a magnetic field between the coils oftransformer 114A. In some situations, particularly where large currents(e.g., such as prescribed by the IEEE 802.3at standard) are supported,transformer 114A may become saturated, significantly degrading thequality of the data signal transmitted on the signal pair 191, 192,increasing bit error rates and instances of packet loss; additionally,link stability may be jeopardized, possibly resulting in link loss insome circumstances.

For instance, where a desired magnitude for currents i₁ and i₂ is about350 mA or greater, even a small percentage difference in R₁ and R₂ mayresult in a difference between i₁ and i₂ on the order of about 100 mA.The resulting residual current may significantly reduce the inductanceof the magnetic coil on the line-side of transformer 114A. As aconsequence, the reduced inductance produces a high-pass filter effecton the data signal to be transmitted via twisted pair 191, 192 throughtransformer 114A. A significant portion (generally at lower frequencies)of the signal sought to be transmitted by PSE device 110 is reflectedback as echo, creating poor signal to noise characteristics,particularly in full-duplex communications mode.

Echo canceller 113 may be employed to reduce some of the effects of aninduced magnetic field at transformer 114A. In particular, echocanceller 113 may generally be operative to identify and remove (from areceived data signal) data signals that were transmitted from PHYtransmitter 111A (echoes) such that what is received at PHY receiver112A is only that signal transmitted by DTE device 120. Echo canceller113 may employ adaptive echo cancelling techniques, for instance, basedupon knowledge of the data signal transmitted by PHY transmitter 111A.When magnetic saturation reflects transmitted energy due to reducedinductance at transformer 114A, echo canceller 113 may remove suchreflected energy using any of various echo cancelling strategies. Itwill be appreciated that echo canceller 113 may also be implemented in asimilar manner to remove energy transmitted by PHY transmitter 111B thatis reflected when transformer 114B is saturated or otherwise suffersfrom reduced inductance. In some implementations, it may be desirable toprovide each respective transceiver pair (i.e., 111A and 112A, on theone hand, and 111B and 112B, on the other hand) a respective dedicatedecho canceller 113. In such embodiments, PSE device 110 may includemultiple echo cancelling hardware devices or functional blocks.Additionally, it will be appreciated that DTE device 120 may employ oneor more echo cancellers (not shown in FIG. 1 for clarity) and tapmonitors (described below) in a similar manner as that set forth withreference to PSE device 110.

For example, a static strategy of echo cancellation may simply subtracta portion of the transmitted data signal from a received data signal;the net result of such subtraction should be a “net” received signal,i.e., the signal transmitted from DTE device 120 with any contributionsof the signal transmitted by PSE device 110 removed. In common practice,a more sophisticated hybrid strategy may be employed in accordance withwhich dedicated circuitry may cooperate with the digital signalprocessing (DSP) operations of echo canceller 113 to eliminate, from areceived signal, any echo associated with a transmitted signal.

In that regard, FIGS. 2A and 2B are simplified graphs plotting tapadaptations in an echo canceller. The curves plot echo energy on achannel as a function of time. It will be appreciated that echocanceller 113 may generally employ a plurality of taps, or delays. Arespective copy of the received signal, delayed by a predetermined timeincrement, may be processed at each respective one of the plurality oftaps. Scaled or weighted signals at each tap may be subtracted from thereceived signal, removing echo energy.

The plot in FIG. 2A illustrates a situation in which i₁=i₂ in FIG. 1,and no (or only a minimal) induced magnetic field exists at theline-side magnetic coil in transformer 114A. The small area 201 of thecurve represents echo energy that is effectively handled by echocanceller 113. In contrast, the plot in FIG. 2B illustrates anunbalanced current condition in which transformer 114A produces ahigh-pass filter effect with respect to twisted pair 191, 192. The largearea 299 of the curve represents echo energy that taxes the capabilitiesof echo canceller 113. The signature large area 299 beneath the abscissain FIG. 2B indicates that an induced magnetic field has saturated, or isthreatening to saturate, transformer 114A.

In accordance with some embodiments, a system and method of determiningPoE impairment may leverage this signature large area 299 by monitoringthe adaptation of taps in echo canceller 113. For example, the areabeneath the abscissa may be integrated; computations resulting in areasabove a certain threshold may be interpreted as indicating a saturationcondition, whereas computations resulting in areas below certainthreshold (which may be different) may be interpreted as an indicationof operation within normal parameters.

In that regard, a tap monitor 115 may be implemented in cooperation withecho canceller 113 to monitor the operation of echo canceller 113. Insome instances, tap monitor 115 may monitor the tap adaptations as setforth above with reference to FIGS. 2A and 2B. Tap monitor 115 may beimplemented as hardware (such as embodied in an ASIC, for instance) oras software instruction sets. In an embodiment in which tap monitor 115is integrated with echo canceller 113, for example, it may be desirablethat tap monitor 115 is a hardware implementation. Alternatively, tapmonitor 115 may be incorporated into or used in conjunction with amulti-purpose microprocessor or microcontroller. In use, tap monitor 115may be apprised of the ongoing operation of echo canceller 113 and mayexamine the function of tap adaptations to identify areas representativeof echo energy above a particular threshold. This threshold may bedevised from empirical data, for instance, or may be predetermined basedupon theoretical or expected values; additionally or alternatively, itmay be desirable to implement the threshold as a selectively ordynamically adjustable threshold. In one such embodiment, a user ornetwork administrator may be enabled to alter the threshold inaccordance with desirable or experienced operational characteristics ofthe network. In another embodiment, tap monitor 115 may receiveadditional input from another component of PSE 110, for instance from atransceiver pair (111A/112A or 111B/112B), a microprocessor or othercontroller, or software 116 (described below); accordingly, tap monitor115 may also be apprised of bit error rates, packet loss, or otherrelevant communications parameters such that the threshold may beadjusted in real time based upon current operating conditions. Inoperation of PSE 110, output from tap monitor 115, either in isolationor in conjunction with other processing steps, may be employed totrigger an alert as set forth below.

Upon determining that a particular energy threshold has been reached orexceeded, tap monitor 115 may trigger a warning event. In oneembodiment, tap monitor 115 may set hardware registers at PSE device 110(or cause such registers to be set); additionally or alternatively, tapmonitor 115 may generate, or cause to be generated, one or more softwareinterrupts. These register settings or interrupts (or some otherequivalent trigger mechanism) may generally be indicative of a magneticsaturation condition at transformer 114A, and may be received orretrieved by higher-level software 116 for additional operations.Software 116 may alert a user or network administrator of the condition,for example, and may additionally recommend corrective action to rectifythe unbalanced current at the source of the condition. As noted above,such corrective action may include ensuring that RJ-45 contacts areclean, changing the LAN cable, or reducing current levels. Also as notedabove, the foregoing components and functionality may be implemented atDTE device 120 in a manner similar to that described with reference toPSE device 110.

FIG. 3 is a simplified flow diagram illustrating general operation ofone embodiment of a method of determining signal impairment due to anunbalanced current condition. The sequence of operations depicted inFIG. 3 may be performed by a PSE device 110 or a DTE device 120 asdescribed above with reference to FIG. 1, for example, or by anothersuitably configured PoE compatible device.

As indicated at block 301, an embodiment of a method of determiningsignal impairment may begin with a PSE transmitting a data signal and DCpower. This transmission generally involves coupling a PHY layer to anetwork cable using appropriate hardware connectors; in one embodimentdescribed above, the transmitting comprises coupling a PSE to a Cat5cable using an RJ-45 connector.

Echo cancellation may be performed as indicated at block 302. TypicalPSE devices employ various types of echo cancellation to improve signalto noise ratios. In accordance with one aspect of the present invention,a method may monitor operation of an echo canceller associated with thePSE device (block 303) to identify a signature representative ofmagnetic saturation (block 304) at a transformer at the connectionbetween the PHY layer and the line-side of the network cable. Asdescribed above with reference to FIGS. 1, 2A, and 2B, this monitoringmay comprise monitoring the tap adaptations of the echo canceller insome embodiments. In operation, DSP algorithms at a tap monitor, theecho canceller, or both, may be employed to integrate echo energy overtime to identify conditions that are characteristic of magneticsaturation. In the foregoing manner, the method may determine when echoenergy reflected back to the PSE is above a predetermined or dynamicallyvariable threshold.

Finally, the method may trigger an alert (block 305). This alert may beoperative to inform a user or network administrator that an unbalancedcurrent condition may be causing magnetic saturation that may adverselyaffect communications signaling. As set forth above, hardware registersettings or software interrupts may be employed to enable software orother instruction sets to generate the alert; as an alternative, analert may be solely hardware-based, in which case one or more bits in ahardware register may be set as an indication of a fault condition, andthe alert may be triggered by this alone. Responsive to the alert beingtriggered, an output may be provided. For example, the alert may includean audible alarm, for example, or a visual display. In some instances, arecommendation may be supplied along with the alert; for example, themethod may recommend, among other things, that contacts be cleaned, thatthe network cable be replaced, or that current be reduced.

In one embodiment, the trigger operation at block 305 may be responsiveto the identification and determination operation at block 304. In theFIG. 3 embodiment, however, optional additional factors may beconsidered as indicated at decision block 311. A determination may bemade at block 311 whether additional factors may affect the triggeroperation. If no additional factors are involved, flow goes directly toblock 305 and the alert is generated. If additional factors areinvolved, flow goes to decision block 321, where a determination may bemade regarding whether certain criteria are satisfied. For example,minimum or maximum bit error rates or packet loss parameters may beexamined, and a decision to generate an alert at block 305 may beinfluenced not only by reflected energy, but also from these or otherreal time conditions of network communications. As one example, if amaximum bit error rate threshold has been set, and a determination ismade that the threshold has been exceeded, then that criterion has beensatisfied; flow would proceed from decision block 321 to triggeroperation at block 305. If, however, the bit error rate threshold hasnot been exceeded, then flow may loop back to block 303. Any of variousadditional factors or communications parameters may be considered atblocks 311 and 321, and these may be application-specific or otherwiseinfluenced by the system in which the PSE is deployed.

It is noted that the arrangement of the blocks in FIG. 3 does notnecessarily imply a particular order or sequence of events, nor is itintended to exclude other possibilities. For example, the operationsdepicted at 303 and 304 or at 304 and 305 may occur substantiallysimultaneously with each other; similarly, the determinations made atdecision blocks 311 and 321 may be incorporated in a single operation,or may be eliminated in some instances.

Several features and aspects of the present invention have beenillustrated and described in detail with reference to particularembodiments by way of example only, and not by way of limitation. Thoseof skill in the art will appreciate that alternative implementations andvarious modifications to the disclosed embodiments are within the scopeand contemplation of the present disclosure. Therefore, it is intendedthat the invention be considered as limited only by the scope of theappended claims.

What is claimed is:
 1. A method of determining an impairment in a Powerover Ethernet application, said method comprising: monitoring operationof an echo canceller with a tap monitor, the echo canceller beingassociated with a physical device, the echo canceller configured toremove echo energy from a signal; measuring the echo energy over time;determining that the measured echo energy over time is above a thresholdindicating a saturation condition; and responsive to said determiningthat the measured echo energy over time is above the threshold,triggering an alert indicating that the saturation condition is causedby an unbalanced current.
 2. The method of claim 1, wherein saidmonitoring comprises monitoring tap adaptations of the echo canceller toidentify areas representative of the measured echo energy being abovethe threshold.
 3. The method of claim 1, wherein said triggeringcomprises setting one or more bits in a hardware register.
 4. The methodof claim 1, wherein said triggering comprises generating a softwareinterrupt.
 5. The method of claim 1, wherein the alert comprises anaudible alarm or a visual display.
 6. The method of claim 1, wherein thethreshold is predetermined based on theoretical or expected values ofthe echo energy.
 7. The method of claim 1, wherein the thresholdcomprises a selectively or dynamically adjustable threshold.
 8. Themethod of claim 1, further comprising adjusting the threshold in realtime based on a communications parameter associated with the physicaldevice.
 9. The method of claim 8, wherein the communications parametercomprises a bit error rate.
 10. The method of claim 1, wherein the alertincludes a recommendation to clean contacts, replace a network cable, orreduce current levels.
 11. A device for use in a Power over Ethernetapplication, the device comprising: an echo canceller to remove echoenergy from a signal received at a receiver; and a tap monitorconfigured to: monitor operation of said echo canceller; compute a valueof the echo energy over time; and responsive to a determination that thecomputed value of the echo energy over time is above a threshold, whichindicates a saturation condition, trigger an alert to indicateunbalanced current as a cause of the saturation condition.
 12. Thedevice of claim 11, wherein said tap monitor is configured to monitortap adaptations of said echo canceller to identify areas representativeof the computed value of the echo energy over time being above thethreshold.
 13. The device of claim 11, wherein the output from said tapmonitor is employed to set one or more bits in a hardware register atthe device.
 14. The device of claim 11, wherein the output from said tapmonitor is employed to generate a software interrupt at the device. 15.The device of claim 11, wherein the alert comprises an audible alarm ora visual display.
 16. The device of claim 11, wherein the threshold ispredetermined based on theoretical or expected values of the echoenergy.
 17. The device of claim 11, wherein the threshold comprises aselectively or dynamically adjustable threshold.
 18. The device of claim11, wherein the threshold varies, in real time, in accordance with acommunications parameter.
 19. The device of claim 18, wherein thecommunications parameter comprises a bit error rate.
 20. The device ofclaim 11, wherein the alert comprises a recommendation to cleancontacts, replace a network cable, or reduce current levels.